Commercial digital wireless systems employ methods which improve the performance of voice transmission and reception. These digital wireless systems serve to mitigate the major problems encountered by users of analog wireless systems such as static, loss/interruption of signal when passing between cells, and failure to get a connection because of congested relays. This is true for systems which are based on digital wireless standards such as Time Division Multiple Access (TDMA), Call Division Multiple Access (CDMA), Global System for Mobile Communications (GSM), Personal Communications Service (PCS), and so forth. Indeed, the majority of service providers of commercial digital wireless systems generate the bulk of their revenue from subscribers using the standard voice service provision.
However, in addition to voice communication, there is an increasing desire by service subscribers to send digital data, such as computer data, video data, facsimile (FAX), and so forth, using digital wireless services. Unfortunately, data and FAX transmission services are not always provided by the digital wireless service providers.
When data and FAX transmission services are offered by the digital wireless service providers, they generally require special interfaces to the subscriber equipment to be able to bypass the vocoders used for voice transmission. The special interfaces to subscriber equipment are not always consistent between service providers in various geographic regions. This inconsistency becomes a problem for subscribers of digital wireless services to be ensured data or FAX services in some geographic regions.
In addition, even when the vocoder system is bypassed, a re-transmit data transmission protocol may cause problems with transmission of "real-time" data. Real-time data is that data which is to be transmitted by the digital wireless service as rapidly as the data is being input into the system. A re-transmit protocol causes data to be re-transmitted by the transmitting end of the communication system when the data is not successfully received at the receiving end of the communication system. While a re-transmit protocol may work when transmitting buffered data files, the re-transmit protocol does not work effectively when transmitting real-time data because some subsequent real-time data may get discarded or delayed when performing the re-transmission operation.
Conventional digital wireless services generally employ vocoders for voice transmission and reception. A vocoder, or voice coder, usually consists of a speech analyzer (encoder) and a speech synthesizer (decoder). The encoder converts analog speech waveforms into digital signals for transmission over a communication path. The decoder converts the received digital signals into artificial (i.e. synthesized) speech waveforms. In addition, the vocoder may be used in conjunction with a key generator and an encryption device to transmit digitally encrypted speech signals over normal narrowband voice communications channels.
Since vocoders are used for voice communications in many conventional digital wireless services, it is desirable to use vocoders for data and FAX transmission as a means of providing data transmission without significantly altering the infrastructure of the digital wireless systems. Unfortunately, vocoders are designed convert analog speech waveforms, rather than the seemingly random bit patterns of a digital data stream. Since the data and fax bit patterns appear random to the vocoders and are not bandwidth limited to audio frequencies, the vocoders do not properly convert the bit patterns prior to transmission.
In general, vocoders employ lossy compression techniques to compress the digitized analog speech waveforms. In other words, a vocoder encoder converts the analog speech waveforms, collects the digitized linear audio (voice) samples, and then compresses the samples using an encoding algorithm. Lossy compression is employed by vocoders to reduce space, bandwidth, cost, and time for the generation, transmission, and storage of voice data by eliminating redundancy, removing irrelevancy, and using various forms of special coding. Thus, lossy compression reduces the amount of data used to represent a given amount of speech information. Rather, lossless compression increases the amount of data that can be stored in a given domain by employing such techniques as simply squeezing a given amount of data into a smaller space.
The vocoder decoder can synthesize the speech waveforms from the compressed data despite the loss of some of the informational content of the voice samples. However, the vocoder decoder does not precisely reconstruct the lost data. Although, lost data can be tolerated when synthesizing speech waveforms, lost data generally cannot be tolerated when transmitting digital data or FAX since every element, or bit, of data carries some meaning within the context of that digital data or FAX stream.
Moreover, vocoders are not compatible with the high data rates of digital data and FAX transmissions. In other words, if the conventional audio modem tone is used to transmit data through a vocoded channel, tremendous data rate reduction occurs compared to a wireline voice channel. The data rate reduction undesirably delays real-time data transmission, monopolizes the vocoder channel which drives up the "airtime" cost of data transmission, and prevents the channel's use for subsequent communications.
Thus, what is needed is a system and method that can be readily employed by service subscribers of digital wireless and other communications services that employ lossy compression techniques to provide data and FAX transmission services where only voice service may be offered. Furthermore, what is needed is a system and method that allows the service subscribers to utilize the standard digital wireless voice services or other lossy compression services to obtain data or FAX services without changes to the elements of the system infrastructure. In addition, what is needed is a system and method that provides reliable, real-time transfer of the data and FAX transmissions.